Revert "[flang] IEEE_ARITHMETIC must imply USE IEEE_EXCEPTIONS"
This reverts commit
b6a7600491d755f6f700fe245b2fc7b928264c58. It caused
the following build failure:
```
ninja: error: dependency cycle: include/flang/__fortran_ieee_exceptions.mod -> include/flang/__fortran_ieee_exceptions.mod
```
See e.g.:
* https://lab.llvm.org/buildbot/#/builders/172/builds/9595
To reproduce:
```
cmake -G Ninja \
-DLLVM_TARGETS_TO_BUILD=host \
-DCMAKE_BUILD_TYPE=Release \
-DLLVM_ENABLE_PROJECTS="clang;flang" \
../../llvm
ninja check-flang
```